Electric filters are well known in the art and various different configurations are known which provide different frequency responses. Typically, an electric filter can be categorized as any of a low pass (by which low frequency signals are passed), high pass (by which high frequency signals are passed), band pass (by which only signals within a certain frequency band are passed), or band stop (by which only signal frequencies outside a particular band are passed). Moreover, within these classifications various other classifications can be made dependent upon the precise filter response, for example Butterworth, Chebyshev, Bessel, or the like. Such filters are well known in the art.
Such filters may be passive filters, by which is meant that the components have no amplifying/active (either voltage or current) elements therein, or active filters, which usually comprise one or more transistors provided with a power supply to act as an active element. Various different transistor circuits are well known in the art, one of which is the source follower circuit (known as an emitter follower, or common collector, when using bipolar junction transistors). The source follower is a well-known basic building block for micro electronics designs, and exhibits excellent linearity even with lower overdrive voltages (Vov=Vgs−Vth). Due to this linearity it has been proposed previously that the source follower circuit can be the basis of a high linearity and low power analogue filter.
More particularly, in D'Amico et al. “A 4.1 mw 10 MHz Fourth Order Source Follower Based Continuous Time Filter with 79. Decibels DR”, IEEE Journal of Solid State Circuits, Volume 41, no. 12, December 2006, there is described a source follower based second order filter based upon a “bi-quadratic cell” architecture. The bi-quadratic cell second order filter disclosed in this document is shown in FIG. 1. Additionally, the same circuit is also described and claimed in WO 2007/034222, which is a prior published International patent application belonging to the present assignee.
The bi-quadratic cell structure shown in FIG. 1 is a pseudo differential structure using a source follower loaded by a positive feedback network. The key element of the bi-quadratic cell is the positive feedback in MOS devices M2 and M3, which allows the synthesis of two complex poles. The arrangement provides significant advantages, such as having extremely good linearity which is achieved for a low overdrive voltage. Additionally, the circuit has low power consumption for a given pole frequency, and a low output impedance is achieved, with no common mode feedback being required. As mentioned, the positive feedback provides for complex poles to be synthesised.
The bi-quadratic cell noted in FIG. 1, and as disclosed in WO 2007/034222 represents a single second order filter. To achieve higher order filters using the prior art bi-quadratic cell, D'Amico et al. proposes cascading multiple cells of the second order, to provide higher, even order, filters. FIG. 2, which corresponds to FIG. 6 of the IEEE Journal of Solid State Circuits article referenced previously, represents a fourth order filter, produced by cascading the output of a first bi-quadratic cell of FIG. 1, with an input of a second bi-quadratic cell. It should be noted, that in FIG. 2 the transistors of the bi-quadratic cell on the left hand side of the figure, i.e., the input cell are PMOS transistors, whereas the transistors of the second, cascaded bi-quadratic cell are NMOS transistors. It should be noted that in the fourth order filter of FIG. 2, which comprises two second order bi-quadratic cells, a total of four transistors are present in each second order cell, being the source follower transistors at the inputs, loaded with the positive feedback network devices (M2 and M3 in FIG. 1). Thus, to achieve the fourth order filter of FIG. 2, a total of eight transistor devices are required.
However, the cascaded arrangement of FIG. 2, while providing a higher order source follower based filter, and hence incorporating the advantages of the source follower arrangement, has two principle drawbacks. Firstly, because the filter essentially comprises two second-order bi-quadratic cells, the transistor component count is relatively high, as essentially each second order cell requires an input stage, as well as the feedback network.
Additionally, the cascaded arrangement is sensitive to component variation, and in particular, when the arrangement is implemented as an integrated circuit. This means that small changes in component characteristics impacts negatively on filter performance in that either the desired transfer function may not be obtained, or the linearity or low power characteristics are not maintained (mainly the transfer function accuracy would be affected). These drawbacks of the cascading bi-quadratic cell structure therefore mean that while it is suitable for use in some applications, it would be preferable if the frequency response accuracy sensitivity could be improved, such that the filter response is rendered more robust to component variations. However, the advantages that it provides which follow from the use of the source follower are particularly desirable, and hence, it would be advantageous if such advantages could be obtained in a structure which does not possess the disadvantage of high sensitivity to component variation.